If you don't remember the significance of Fifth Avenue in the Trumpian mythos, here's a refresher: It came early in the 2016 presidential campaign, when the future president was riding high in the polls and already demonstrating an ability to shrug off controversies that would have destroyed other, more normal, candidates.

And so he exulted in Iowa: "I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn't lose voters."

It was an alarming claim.

Unfortunately, there has been little since then to prove him wrong. Trump has insulted veterans, survived the Access Hollywood tape in which he bragged about forcefully groping women, disparaged minorities, and equivocated about Nazis. Yet he has survived, unscathed. Sure, his approval rating is still well below 50 percent in most polls, but he has 90 percent support from Republicans. Given how he won the presidency while losing the popular vote, he may figure that's all he needs to stay in the Oval Office. The uglier Trump gets, the more the GOP rank and file seem to love him.

But there is one thing that could crack Trump's political support: his trade policies.

He isn't opening fire on Fifth Avenue, but through his trade wars — for which he reportedly has no end game — Trump could very well inflict pain and suffering on many of his supporters. Their jobs and lives are in danger, and this might be the clearest possible test of their loyalty to him.

This would seem to present a political problem for Trump. Iowa and Ohio, for example, are both bellwether states that went Republican in 2016. They're also among the country's top six producers of soybeans, the leading agricultural export in 2017.

Soybeans are also on China's list for retaliatory tariffs.

"The math is simple," said John Heisdorffer, the president of the American Soybean Association. "You tax soybean exports at 25 percent, and you have serious damage to U.S. farmers."

That's why elite Republican politicians from farm states — often silent about Trump's foibles — have found their voices to oppose the president's trade isolationism. After all, they have to face the voters too.

"It's not fair, and it doesn't make economic sense," Sen. Charles Grassley, an Iowa Republican, complained in April. Even Trump's own agriculture secretary has made warning sounds.

Despite all this, the president doesn't seem concerned. "I tell you, our farmers are great patriots," he said back in April. "They understand that they're doing this for the country. And we'll make it up to them."

Surely, this is a bridge too far. Trump can't damage the livelihoods of his supporters and still get their vote in 2020, could he?

Actually, he could.

Last week, theThe Kansas City Stargauged the situation in Poplar Bluff, Missouri, where Mid Continent Nail Corp. has shed 60 jobs in the last month, and may have to shut down entirely because of Trump's tariffs. It found Trump's popularity there intact. He's "better than the Muslim we had in the White House," one resident told The Star.

So much for economic anxiety.

Trump's trade war might be worth it if there were any evidence it was likely to produce more benefits than losses. Alas, the president appears to have no end game planned — just escalation and retaliation until one side or the other blinks. That means we as a nation are standing in the middle of Fifth Avenue — and Trump, armed with little more than bravado and an oversized ego, is taking aim. Any other president would probably lose voters. Will he? Or is Trump completely impervious to the laws of political reality? We're about to find out.